Aromatic acids comprise at least one aromatic ring, typically a benzene or naphthalene ring, substituted by at least one carboxylic acid group. Examples of aromatic acids include phthalic acid, isophthalic acid, terephthalic acid, 2,6 naphthalene dicarboxylic acid, and benzoic acid. When reacted with other monomers such as diols (e.g. ethylene glycol), aromatic acids may be used to make useful polymers such as polyesters (e.g. polyethylene terephthalate). These resulting polyesters are useful in a variety of applications including containers, films, packaging materials, fibers, and others.
Aromatic acids are typically manufactured by an aromatic oxidation process wherein a feedstock comprising an aromatic compound substituted with at least one oxidizable group such as alkyl or acyl group or combinations thereof is oxidized to form a crude aromatic acid. Typical feedstocks suitable for oxidation to form aromatic acids include ortho-xylene, meta-xylene, paraxylene, 1,5 dimethylnaphthalene, 2,6 dimethylnaphthalene, and the like. The feedstock is typically oxidized in a reactor in the presence of a carboxylic acid solvent, oxidation catalyst, and a source of oxygen. The catalyst used in the oxidation process typically comprises one or more oxidation catalyst metals, including those metals having an atomic number of about 21 to about 82.
The aromatic oxidation process is typically an exothermic oxidation reaction, which results in the formation of a crude aromatic acid product in an oxidation reactor. Typically, the crude aromatic acid precipitates to form an oxidation slurry with a solid phase comprising precipitated crude aromatic acid product and an oxidation liquid stream. The oxidation liquid stream comprises the carboxylic acid solvent, water, and various materials in solution including unreacted feedstock, unprecipitated crude aromatic acid product, unprecipitated oxidation reaction by-products and oxidation catalyst materials, e.g. cobalt, manganese and bromine. The crude aromatic acid product may be separated from the oxidation liquid stream by subjecting the oxidation slurry to a solid-liquid separation step. Once separated from the crude aromatic acid product, the oxidation liquid stream is often also termed as “oxidation mother liquor.” All or a portion of this oxidation mother liquor is frequently recycled, i.e. returned, to the oxidation reactor.
The crude aromatic acid product is typically purified in an aromatic acid purification process wherein the crude aromatic acid product is dissolved in water and treated with hydrogen and a hydrogenation catalyst under elevated temperature and pressure. After temperature and pressure are reduced, the aromatic acid purification process yields a purification slurry with a solid phase comprising precipitated purified aromatic acid product and a purification liquid stream. The purified aromatic acid may be separated from the purification liquid stream by subjecting the purification slurry to a solid-liquid separation step. Once separated from the purified aromatic acid product, the purification liquid stream is often referred to as a “purification mother liquor.” The purification mother liquor usually is predominantly water and typically comprises minor amounts of additional components such as soluble hydrogenation by-products and, when purification is conducted as part of an integrated aromatic acid manufacturing process comprising oxidation and purification steps, may also contain residual carboxylic acid and minor amounts of oxidation catalyst metals. Purification mother liquor, or a portion thereof remaining after removal of soluble by-products, frequently is recycled in whole or in part to the process.
Certain problems are encountered during the aforementioned aromatic oxidation process and aromatic acid purification process which result from the contamination of liquid streams with dissolved iron. This dissolved iron contamination typically results when liquid streams are directed and exposed to iron-containing surfaces of equipment used during these processes. For example, oxidation and/or purification mother liquor is typically directed and exposed to iron-containing surfaces of equipment. Problems associated with dissolved iron contamination may be ameliorated by decreasing the exposure of liquid streams to iron-containing surfaces of equipment, such as by alternative use of solid titanium or titanium-clad equipment. Nevertheless, because of the relatively high expense of titanium, the exposure of liquid streams to iron-containing surfaces of equipment (e.g. stainless steel) remains a typical occurrence. Examples of equipment having iron-containing surfaces include pumps, transfer lines, vessels, and the like. Dissolved iron contamination is undesirable because of its potential to precipitate as iron oxide. Accumulation of iron oxide over time will typically begin to negatively affect the usefulness of a piece of equipment. For example, accumulation of iron oxide on the surface of titanium cladding may promote accelerated corrosion. Accordingly, it would be desirable to discover a method for removing dissolved iron from oxidation and/or purification liquid streams.
The problems associated with precipitation of iron from liquid streams contaminated with dissolved iron may be better understood with reference to the effects on particular equipment items. The aromatic oxidation process involves an exothermic reaction typically producing an off-gas comprising vaporized solvent and vaporized water. This off-gas or a portion thereof can be directed to a distillation column to separate the solvent from the off-gas so that it may be recycled. Passing through the distillation column, the off-gas is cooled while contacting internal packing materials or trays. This cooling allows the lower boiling point components, such as water, to be removed from the top of the column, while the higher boiling point components are returned to the bottom of the column and can be re-used, for example as solvent for the oxidation reaction. The cooling is typically assisted by introduction of a reflux at the top of the distillation column. This reflux typically comprises a liquid stream (preferably aqueous) containing materials which are the same as, or compatible with, the components of the oxidation process. Examples of such a liquid stream comprise water condensed from the distillation overhead gas, or from a predominantly water stream obtained by first condensing oxidation reactor off-gas to separate carboxylic acid solvent and subsequently condensing a portion of a resulting gas stream or, in processes in which oxidation and purification steps are integrated, a purification mother liquor resulting from the separation of a purification liquid stream from purified aromatic acid product or product and soluble purification by-products. Using such a purification mother or other liquid streams which may contain dissolved iron due to contact with equipment surfaces composed of iron or steel in the reflux may contribute to the formation of solid iron oxides on the surface of the internal packing materials of the distillation column.
The accumulation of iron oxides on packing materials comprising titanium is particularly undesirable. One publication has concluded that: “Accumulations of iron oxide . . . on titanium structured packing can promote or accelerate combustion of titanium. It may be appropriate to periodically remove accumulations of such materials through chemical or other means.” (Centerline, Vol. 5, No. 2, Summer 2001, pp. 6–8, 15–18, published by Mary Kay O'Connor Process Safety Center). This publication further reports on a safety incident involving a fire at a chemical manufacturing facility, concluding that the presence of iron oxides “accelerated the oxidation of the titanium [packing materials] via a mechanism known as the Thermite Reaction in which the oxygen for combustion is taken from a less reactive metal oxide.”
In U.S. Patent Application No. 60/327,464 filed Oct. 5, 2001, a cleaning process is proposed to remove accumulated iron oxides from the surface of aromatic acids manufacturing equipment exposed to liquid process streams which may carry dissolved iron. Nevertheless, it would be desirable to eliminate or decrease the need for cleaning by discovering a method of removing dissolved iron contamination from liquid streams during the aforementioned oxidation process and/or purification process.